I associate Agatha Christie with the 30s and 40s, sometimes forgetting the first decade of her work. As a teenager, I decided to read her books in publication order so I've read her 1920s output, but for the most part haven't re-read those novels. There may be a reason for that - if The Secret Adversary is exemplary, it took her a few years to find her voice.
Childhood friends Prudence "Tuppence" Crowley and Tommy Beresford meet by chance shortly after being discharged from national service into very Christie-like genteel poverty. Over lunch, they decide to start a business as the Young Adventurers - willing to do anything, legal or illegal, for the right price. They're overheard by a stereotypically sinister man who follows Tuppence and offers her a job which he withdrawals when she gives the pseudonym "Jane Finn." Naturally, Tommy and Tuppence decide to find the real Jane Finn, a young American woman who apparently received important papers as the Lusitania sank. They meet a member of the British Secret Service, a man who may be the next Prime Minister, and Jane's American cousin; Tommy gets kidnapped; Tuppence goes into service for the woman they think will lead them to Jane Finn, and in the end, it's Tommy's plodding nature rather than Tuppence's quicker wits which save the day. It's a quick read, and Christie creates plausible streams of evidence for both main suspects.
What Christie does not do is create believable characters. That was never her strong point, but having recently re-read several of her later works, it's particularly jarring here. The criminals Tommy meets are rough, ethnic stereotypes and the main characters don't fare much better. The Young Adventurers - particularly Tommy - are so "pip pip tally ho!" that as I read, images of a young Hugh Laurie playing one of his upper-class twits kept flitting across my mind, and I heard the American Julius Hersheimmer's lines as spoken by Graham Chapman playing a movie studio head. They're cardboard cutouts created to serve the plot. Lucky for Christie that she constructed such a clever puzzle.